Techniques for monitoring ligand-receptor interactions are vital to a wide number of fields ranging from biotechnology to fundamental cell biology. Moreover, the associated sensing technologies may be of direct interest to the armed forces (e.g., the U.S. Navy). Such measurements are often made by fluorescently labeling proteins and nucleotides of interest. Indeed, fluorescent tags have become a ubiquitous tool for detecting protein-ligand interactions. (Soper, S. A., et al., Anal. Chem. 70:477 R-494R, 1998.) Labels can, however, interfere with the detection process and be extremely cumbersome to implement with real-time sensor devices. (Tan, P. K., et al., Nucleic Acids Res. 31:5676-5684, 2003.) This has been a major driving force behind the creation of assays that can detect biological analytes without labeling them (i.e., label-free detection). Methods for label-free detection include the use of liquid crystalline phase transitions (Kim, S. R. and N. L. Abbott, Adv. Mater. 13:1445-1449, 2001; Brake, J. M., et al., Science 302:2094-2097, 2003), colloidal particle imaging (Baksh, M. M., et al., Nature 427:139-141, 2004), semiconductor nanowire conductivity (Cui, Y. et al., Science 293:1289-1292, 2001; Patolsky, F., et al., Anal. Chem. 78:4260-4269, 2006; Wang, W. U., et al., Proc. Natl. Acad. Sci. U.S.A. 102:3208-3212, 2005), quartz crystal microbalance (QCM) measurements (Muratsugu, M., et al., Anal. Chem. 65:2933-2937, 1993; Cooper, M. A., et al., Nat. Biotechnol. 19:833-837, 2001; Yao, C. Y., et al., J. Nanosci. Nanotech. 6:3828-3834, 2006), and surface plasmon resonance (SPR) spectroscopy (Yang, C. Y., et al., Lab on a Chip 5:1017-1023, 2005; Kroger, D., et al., Anal. Chem. 71:3157-3165, 1999; Hoffman, T. L., et al., Proc. Natl. Acad. Sci. U.S.A. 97:11215-11220, 2000). These techniques, however, are not always easy to employ, can give a nonlinear response to the analyte, require specialized equipment, and/or suffer from poor sensitivity in comparison with fluorescence-based measurements (Song, X. D. and Swanson, B. I., Anal. Chem. 71:2097-2107, 1999).
SPR has perhaps become the most popular choice for label-free detection. This measurement, which is based upon changes in refractive index, requires a metal coated substrate (e.g., Au or Ag) and is performed in most cases by a dedicated and expensive commercial instrument (Markov, D. A., et al., J. Am. Chem. Soc. 126:16659-16664, 2004). The experiment is temperature sensitive and the signal abstracted is not necessarily linearly dependent on surface coverage. To make a measurement, one immobilizes a ligand on the metal substrate and flows a receptor over the surface. Under ideal conditions, the technique has an analyte detection limit in the low picomolar concentration range when the equilibrium dissociation constant for the ligand/receptor interaction is on the order of a few nanomolar (Yang, C. Y., et al., Lab on a Chip 5:1017-1023, 2005). SPR can be run in imaging mode to obtain many binding measurements simultaneously (multiplexing) (Brockman, J. M., et al., J. Am. Chem. Soc. 121:8044-8051, 1999; Wegner, G. J., et al., Anal. Chem. 74:5161-5168, 2002; Lee, H. J., et al., Anal. Chem. 78:6504-6510, 2006); however, the sensitivity is correspondingly poorer. In fact, SPR imaging sensitivity has been found to be on the order of 1 nM of bulk analyte for a 20 nM interaction (Lee, H. J., et al., Anal. Chem. 78:6504-6510, 2006). This has led to the development of subsequent chemical amplification steps in order to detect lower concentrations of analyte in solution (Li, Y., et al., Anal. Chem. 79:1082-1088, 2007). Other methodologies are needed that improve upon these inadequate detection thresholds and other deficiencies associated with label-free detection techniques.